I truly do not think their will be sub classes. While it would not surprise me if Bungie did this (for a more unique character base) i do not think they will. Their may be a "undefined" type of subclass where the fan base (the players) sorta make one (as in all warlocks get a "healing" spell, but some people may make warlocks specially for that).
As for people that say about the Vanguard thing, it was confirmed that that was a type of armor in game, not a subclass.
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The point is though, whether or not there are clearly defined subclasses, Bungie has probably designed the various abillities so that you can create a variety of sub-archetypes off a main class. For example, if they put in a shield abillity for a titan, that encourages a tank archetype, a healing spell encourages a cleric archetype for the warlock, cloaking encourages an assassin archetype for the Hunter, etc.